Fullerenes are well known as omnidirectional electron acceptors. Due to the spherical and symmetrical nature of the more common fullerenes, such as C60 known as Buckminster fullerene, electrons can be accepted at any point on the fullerene cage, which differs from typical organic electron scavengers that accept electrons only at specific sites of the scavenger molecule. This non-specific electron accepting ability of fullerenes has been exploited for various applications including electrochemical sensors, photochemical cells, therapeutics, photocatalysis, and electronic devices such as n-type and field effect transistors.
Pristine and functionalized fullerenes have been employed as electron acceptors in electrochemical sensors for detection of various organo- or bio-analytes such as dopamine and nandrolone. Typically, the electrochemical sensor comprises an electrode upon which fullerenes have been coated. The sensing results due to the transfer of electrons to the fullerene by a negatively charged analyte upon application of an electric field.
For photochemical and solar cells, typically fullerenes are employed as metal porphyrin-fullerene combinations or in other forms such as dyads. For a metal porphyrin-fullerene combination the metal porphyrin generates electrons upon absorption of photons. The electrons are then scavenged by the conjugated fullerenes to promote the flow of current through a circuit. In addition to metal porphyrins, other photoactive materials that can be combined with fullerenes for use in photochemical cells include TiO2, ZnO and various organic dyes.
Fullerenes scavenge free radicals, such as superoxide, by accepting electrons from the free radical. These antioxidant properties of fullerenes have been applied for therapeutic uses, such as the treatment of various diseases such as Alzheimer and Parkinson disease and for reducing the side-effects of cancer treatment. Fullerene comprising compositions have been commercialized as anti-ageing cosmetics.
Fullerenes have been used in combination with semiconductor photocatalysts. Semiconductor photocatalysts are employed for the destruction of environmentally hazardous chemicals and bioparticulates, as these materials can be a cost-effective means to achieve complete mineralization of these environmental hazards without generation of toxic byproducts. For example, titanium dioxide has been commercially applied as a self-cleaning coating on buildings and glass materials. However, such applications have been limited by the low quantum efficiency of these photocatalysts. In photocatalysis, with a photocatalyst such as titanium dioxide, electron-hole pairs are generated in the semiconductor upon irradiation with ultraviolet light. Some of the photo-generated electrons and holes migrate to surfaces, where they undergo redox reactions to generate reactive species such as hydroxyl radicals. However, the photo-generated electrons and holes can also undergo recombination, which reduces the quantum efficiency of photocatalysis.
Several attempts have been made to separate the photogenerated electrons and holes to reduce recombination. Titanium dioxide photocatalysts have been conjugated or doped with electron scavenging agents such as metals or organic molecules. Numerous organic molecules have been conjugated with titanium dioxide for applications that include solar cells and visible light photocatalysis. Platinum, gold and silver metals are often employed as scavenging agents, generally due to their high conductivity, although conflicting results have been reported. Another class of conductors examined is carbon nanotubes. For example, anatase coated multi-wall carbon nanotubes (MWNT) achieved twice the efficacy of a commercial photocatalytic TiO2 (Degussa P25) for inactivation of bacterial endospores. It was hypothesized that the photo-generated electrons are scavenged by the MWNT. These approaches involve processing to conjugate or dope the TiO2 or scavenging agent and have resulted in a relatively high cost to produce the modified photocatalysts.
As indicated above, fullerenes such as C60, because of their unique electronic properties, have been examined for combination with semiconductor photocatalysts. Kamat et al., “Photochemistry on Semiconductor Surfaces. Visible Light Induced Oxidation of C60 on TiO2 Nanoparticles”, J. Phys. Chem. B, 1997, 101 (22) 4422-7, discloses the transfer of photo-generated electrons from titanium dioxide to fullerenes with ethanol/benzene mixed solvent. Fullerenes are extremely hydrophobic, limiting their use for enhancing photocatalysis in aqueous environments. The water solubility of fullerenes improves by forming hydroxyl groups on the surface of the fullerenes. However, addition of hydroxyl groups to the fullerene structure modifies the electronic properties of the fullerenes. The toxic effects attributed to fullerenes are not observed for the hydroxylated form of fullerenes. Rather, polyhydroxyfullerenes are able to reduce oxidative stress on cells by scavenging reactive oxygen species and have been patented as therapeutics as, for example: Chiang et al., U.S. Pat. No. 5,994,410.
Recently Polyhydroxyfullerenes (PHFs) were reported for enhancement of the photocatalytic activity of titanium dioxide (TiO2). Krishna et al., “Enhancement of Titanium Dioxide Photocatalysis with Water-Soluble Fullerenes”, J. Colloid and Interface Sci., 2006, 304, 166-71, demonstrated that hydroxylated fullerenes display electron accepting properties and can be employed to scavenge photogenerated electrons from TiO2, thereby increasing the rate of photocatalysis. However, not all PHFs enhance the photocatalytic activity of TiO2. Hence, identification of the PHF structures that promote TiO2 photocatalytic activity is needed.